Many processes involve gas and particulate solids flows where the solids are recycled back to the process for further use. Examples of such circulating flow of solids are the flow of solid catalyst in a Circulating Fluidized Bed (CFB) reactor and coating of the particles in a cycle spouted bed. In these systems, the solids circulation rate, which affects both heat and mass transport properties and determines the gas-solid contact time and the performance of the reactor, becomes a highly significant operating parameter. However, measurement challenges abound. The system by its nature promotes highly complex interactions between the gas and the particulates, and simple tracer—response methods produce results that are not unique. Attempts to avoid this complexity concentrate on estimation of the time-averaged solids flow rate across a given section, and assume it to be an estimate of the overall solids circulation rate in the closed loop. However, additional challenges are presented. The high temperatures and gas compositions commonly found in these processes are relatively aggressive to intrusive instruments, and the high pressure with particulate laden flows make the sealing of mechanical motion across the pressure boundary difficult. Further, introduction of intrusive instruments may in turn change the flow itself, leading to a systematic error in the measurement. Additionally, the flow response to specific components which may or may not be present among different circulating systems often demands a-priori calibration that is often difficult to perform in situ. All of this is compounded by the fact that operating conditions vary significantly from application to application. For example, for coal combustion, the gas velocity and solids flux are typically 5-8 m/s and less than 40 kg/m2-s, respectively. For fluid catalytic cracking (FCC), on the other hand, the gas velocity and solids flow rate are considerably higher, e.g. 15-20 m/s (at the riser exit) and greater than 300 kg/m2-s, respectively. As a result, a metering technology generically suitable for gas and particulate solids flows is still lacking.
Among the few techniques available for estimating the solids circulation rate in a hot, pressurized solids circulating system, one of the more simple methods involves the measurement of particle velocities at the wall within the packed bed portion of a standpipe. Typically, in a system utilizing circulating solids, the standpipe serves as a component in the solids recirculation loop. In applications involving chemical reactions, the standpipe may also serve as a heat regulator or spent sorbent regenerator. During operations, individual particles are tracked at the wall of the standpipe, and the time needed for an individual particle to travel a known distance is measured in order to determine a particle velocity. This velocity, in conjunction with the cross-sectional area for flow and the solids bulk density, allows a mass circulation rate to be determined. However, this method necessarily assumes that the particles at the wall travel at the same velocity as the bulk, since the presence of the bed material prevents an observer from seeing into the interior of the standpipe. This can introduce significant error. For example, particle slippage due to surface roughness and wall friction can produce a significant velocity deviation between the particle observed at the wall and the bulk flow. Additionally, there may be significant logistical hurdles to this method. For a hot unit at pressure, this technique requires a high temperature window with significant thickness to withstand the operating conditions, which at the same time must be kept sufficiently clean so that individual particles can be seen and tracked.
Another method employed involves calorimetric measurements. Many hot systems have heat exchanger equipment in the packed bed portion of the standpipe to control the temperature of the circulating solids. By measuring the temperature change and flow rate of the heat transfer fluid in the heat exchanger while simultaneously measuring the temperature change of the hot circulating solids, the solids circulation rate can be estimated. Clearly, this method is only applicable to systems incorporating submerged heat exchangers and solids at higher temperatures. Additionally, the method assumes that solids temperature is uniformly lowered by the heat exchanger, and that all of the solids flow through the heat exchanger. However, in practicality, because of heat changes driven by seal air and heat radiation, among other factors, the method requires significant plant specific calibration, especially for large systems. As an example, see “Experimental Study on an On-Line Measurement of High Temperature Circulating Ash Flux in a Circulating Fluidized Bed Boiler,” Lu Xiaofeng, et al, J. of Thermal Science, Vol 10, No. 2 (2001).
Another method for estimating the solids circulation rate is based on gas velocity measurement in a riser. Within the riser of a solids circulating system, the gas travels upwards faster than the solids. The difference between the gas velocity and the solids velocity is called the slip velocity. Knowledge of slip velocity and solids concentration in the riser allows determination of a solids mass circulation rate. However, solids flow dynamics in gas-solid risers is inherently complex, and the solids slip velocity is not a simple function of operating conditions. Any measurement of gas velocity using this method will likely be assuming a plug flow through the riser, which can be significantly at odds with the actual situation. See, e.g., “Investigation on slip velocity distributions in the riser of dilute circulating fluidized bed,” Yang Y.-L, et al, Powder Technology, Vol. 73, pp. 67-73 (1992). Further, solids concentration is not uniform across the diameter of the riser, and shear forces between the riser wall and the particles increase the error in the solids concentration estimate using pressure drop measurements. Additionally, these techniques may introduce invasive probes into the riser, changing the flow itself and leading to a systematic error in the measurement.
Another method for estimating the solids circulation rate is based on pressure drop across a specific part of the equipment or across an orifice. In this method, experimentally measured pressure drop, together with the gas velocity, is correlated with the solids mass flux. This technique is effectively non-interfering with the flow in the riser, however, because different combinations of gas and solids flowrates can lead to the same pressure drop, the solids mass flux must be independently estimated using a time-of-descent method or some other method. See, e.g., “Development of a J-shaped Pneumatic Valve to Control the Solid Particle Circulation rate in a Circulating Fluidized Bed,” Terasaka, K. et al., Powder Technology, Vol 126, p. 13-21 (2002). This necessitates a calibration process which can be arduous for industrial scale equipment.
In order to avoid the issues associated with the aggressive impact of high temperature and gas compositions, the intrusiveness of measuring instruments leading to systematic error, and the difficulty of extensive calibration in large-scale units, flow correlations such as the Ergun equation have been utilized to correlate moving bed flow with relatively easily obtained pressure measurements. The Ergun equation is well known and traditionally used to describe the pressure drop of a liquid or gas flowing through a stationary packed bed. It relates the pressure drop to a specified flow rate, the flowpath length through the bed, the equivalent spherical diameter of the particles in the bed, the density and dynamic viscosity of the liquid or gas, the velocity of the liquid or gas with respect to the bed, and the void fraction of the bed. See, Coulson and Richardson's Chemical Engineering, Richardson, J., et al., Butterworth-Heinemann (2002), among many others. This concept is further extended to moving packed beds where both the liquid or gas and the fixed bed itself are in motion relative to a containing plant component, such as a standpipe in a circulating fluidized bed reactor. For these situations, the Ergun equation and its modified forms utilize the concept of superficial gas velocity, sometimes termed slip velocity, which is simply defined as the relative velocity between the fluid or gas and the moving packed bed. Determination of the superficial gas velocity and separate measurement of the gas velocity with respect to the standpipe is then used to determine bed velocity with respect to the standpipe, and the subsequent solids flowrate. See, Fluidization, Solids Handling, and Processing: Industrial Applications, Yang, W., Noyes Publications (1999), among many others. Typically in practice, the pressure drop over a length of bed is measured while the equivalent particle diameter, gas density, and gas viscosity is estimated, and the superficial gas velocity and void fraction remain as unknown quantities. At that point, the value of void fraction is often further assumed in order to finalize a superficial gas velocity. For example, see “An Analysis of Loop Seal Operations in a Circulating Fluidized Bed,” Basu, P. et al., Trans IChemE, Vol. 78, Part A, p. 991-998 (October 2000); see also “Simultaneous Measurements of Gas-Solid Flow Rates and Pressure Drop in Downcomer of J-Valve in CFB,” Goshima, T., et al, Chem. BioChem. Eng., Q 21 (4), p. 357-363 (2007); and see also “Solids Flow Characteristics in Loop-Seal of a Circulating Fluidized Bed,” Sung Won Kim, et al, Korean J. Chem Eng., Vol 16, p. 82-88 (1999). Another commonly used approach is to express the void fraction as a linear function of the superficial gas velocity, thereby leaving superficial gas velocity as the only remaining unknown. See, e,g., Pneumatic Conveying of Solids: A Theoretical and Practical Approach, Klinzing, G., et al., Springer (1997), among many others. However, this treatment of void fraction, heretofore necessary, can introduce significant error when an Ergun correlation is used to determine a superficial gas velocity.
Mathematically, the void fraction typically appears in Ergun correlations as a cubed term and any errors in the void fraction value have a dramatic effect on mathematically determined superficial gas velocities. For example, a 5% error in the void fraction will produce a 55% error in the slip velocity. Therefore, it is necessary to determine void fraction with a great deal of precision. See Chemical Reactor Design, Harriott, P., CRC Press (2003). In moving bed flow, this is additionally complicated by the fact that the void fraction may not be constant with time, and can change based on changes in operating parameters elsewhere in the system. For example, in a circulating fluidized bed characterized by a riser loop and a recirculating standpipe, under normal operating conditions with a constant mass circulation rate, the pressure drop across the riser is balanced by the pressure drop across the standpipe loop. If a small reduction in gas velocity through the riser takes place, the flow in the riser responds by increasing the pressure drop across the riser. The increase in pressure drop must be balanced by an increase in pressure drop in the standpipe. This increased pressure drop in the standpipe will change the flow of gas through the standpipe and shift the operating value of void fraction in the standpipe. This can introduce further error into the estimated value of void fraction, and further error in the values of superficial gas velocity determined using Ergun correlations.
What is needed is a technique for measuring solids flowrate which mitigates the aggressive impact of the high temperatures and gas compositions often encountered, is non-intrusive to the flow itself so that systematic errors from changes in the flow itself are minimized, is capable of operation in large-scale units with minimized calibration requirements, and is able to cover a broad range of circulation rates with consistent accuracy by determination of void fractions and superficial gas velocities based on operating parameters.
Accordingly, an object of one embodiment is to provide the bed velocity of a moving packed bed in the standpipe of a circulating bed reactor by measuring the differential pressure and gas velocity in a first section of the standpipe over a first axial distance and in a separate second section of the standpipe over a second axial distance, and determining the void fraction and bed velocity based on an Ergun correlation describing the interaction of measured parameters in the first and second sections.
It is another object of one embodiment to provide the bed velocity of a moving packed bed in the standpipe of a circulating bed reactor utilizing a method where the fixed moving bed void fraction is determined based on in-situ measurement of operating conditions, rather than assumed as a constant value or related to other prevailing conditions in the standpipe with empirical relationships, thereby minimizing the impact of void fraction error stemming from initial assumptions or changes in plant operating condition.
It is another object of one embodiment to provide the bed velocity of a moving packed bed in the standpipe of a circulating bed reactor utilizing a method which avoids plant specific calibration, by providing a method whereby operating conditions in an instrumented standpipe section are observed and related to bed velocity with Ergun correlations for gas or fluid flow through a moving packed bed applied to that instrumented standpipe section, thereby avoiding a large number of plant specific variables.
It is another object of one embodiment to provide the bed velocity of a moving packed bed in the standpipe of a circulating bed reactor utilizing measurement techniques that mitigate the aggressive impact of high temperatures and gas compositions in the circulating bed reactor on measurement instruments.
It is another object of one embodiment to provide the bed velocity of a moving packed bed in the standpipe of a circulating bed reactor utilizing measurement techniques that are non-intrusive to the flow itself, mitigating any systematic errors introduced by measurement instruments.